1. Field of the Invention
This invention applies to target acquisition involving location of a target followed by target tracking using infrared imagery; more specifically, the invention is a high-sensitivity infrared detector and an infrared camera using such a detector to fulfil, in particular, target acquisition functions.
Conventionally, infrared cameras include a detector in the form of a strip or mosaic of photosensitive cells arranged, respectively, either in a row of a small number of cells or in a matrix of rows and columns. This detector is placed on the cold face of a cryostat and analyzes an image of the scene received through a lens. An opto-mechanical system scans this image on the detector to generate a signal proportional to the luminous flux received from the scene observed. Examples are the 288.times.4-pixel strip and the 32.times.32-pixel matrix developed by SOFRADIR. These cameras produce a video signal by converting, between two read operations, the charges accumulated in storage sinks which are directly proportional to the flux received by each pixel. The conversion is done by devices known as CCDs (Charge-Coupled Devices). These devices are controlled by an addressing circuit and allow selective access to the storage sinks; they multiplex the charges stored to form the video signal.
Matrix detectors with a considerably higher number of cells have been developed recently. These detectors analyze the image directly without needing a scan system and then form veritable electronic-scan retinas. For example, LIR have developed a 128.times.128-pixel electronic retina.
Generally, infrared cameras are used in homing heads capable of searching for a target and then tracking it, depending on the mission phase.
In the search mode, the camera must scan all or part of the accessible surrounding space. The camera must, therefore, be used with an opto-mechanical scanning system located at input to the homing head, for example a panoramic mirror with a variable line of sight. The scene contents can then vary quickly in time.
In the tracking mode, once the camera has identified or "locked onto" the target, the line of sight varies little since the image processing resources allow it to be slaved to the target direction. The contents of the scene seen then vary little during this second phase.
Regardless of the mode in which the camera is used, high sensitivity is required to correctly analyze the signal output. In fact, the continuous component of the signal, representing the scene background, forms a very large part of the signal compared to the variable component which represents the useful information. For example, in conventional 3-5 .mu.m and 8-12 .mu.m infrared spectrum windows, a difference of 1.degree. in temperature between the scene and the background only changes the video signal 1% compared to the continuous component.
2. Description of the Related Art
In general terms, to improve the sensitivity, it is essential to increase the signal level detected for each unit surface area in the image of the scene observed, generally referred to as the picture element or pixel.
One solution is to increase the period over which the charges in the storage sinks are accumulated but saturation of the sinks considerably limits the efficiency of this method.
Another solution is to increase the number of photosensitive cells for each pixel: it is known that the sensitivity of a detector is proportional to the square root of the number of photosensitive cells used to form a given pixel. However, the image resolution decreases quickly.
These two general methods therefore offer very limited possibilities of improving the sensitivity.
A specific technique has therefore been developed, for each mode of infrared camera use, to further increase the sensitivity of the detectors; this improvement is obtained by following each pixel in time and accumulating the successive charges received for the same pixel.
In the search mode, the image changes quickly in time and the most suitable method of improving sensitivity uses the TDI technique (Time-Delay Integration). TDI involves adding the charges stored, in succession, by the various individual photosensitive cells which receive, at different times, the light flux corresponding to a given point in the scene observed. The processing may be applied at the detector itself (when it is known as internal TDI) or outside the detector (external TDI), using a digital operator and a memory dedicated to this function. TDI can only be applied to scanning detectors whose photosensitive cells "see" the same point in the scene as successive instants and is not, therefore, suitable for static electronic retinas or matrix detectors.
In the tracking mode, sensitivity is improved by a technique known as post-integration. This technique is based on the fact that, because the scene observed varies very little in time, a given individual cell sees the same pixel at different instants. Post-integration then involves summing, pixel by pixel, n successive charges corresponding to n successive images. For a given pixel, the equivalent time over which the charges stored are accumulated is thus increased without saturating the storage sinks. To implement this function requires the use, after digitization, of an image memory and a dedicated operator. Post integration is only applicable to detector retinas or, possibly, detectors used statically, i.e. with no scan.
Consequently, it is only possible to improve the sensitivity of the detectors described above in one homing head operating mode to the exclusion of the other, i.e. by using scanning detectors in the target location mode and static detectors in the target tracking mode.
The problem is to obtain a signal whose sensitivity remains high for both these main phases in a conventional target acquisition function, i.e. during both the search phase and then, after locking onto an identified target, in the target tracking phase.